1. Technical Field
The disclosed invention relates to optical systems used to correct phase distortion in a propagating optical wavefront and, more particularly, to adaptive-optics systems that correct the phase distortion in real time.
2. Description of Related Art
Adaptive-optics systems are used in a variety of applications where the characteristics of optical elements are selectively changed in real time to modify optical wavefronts being received thereon. One such application is with large ground-based telescopes imaging objects in space, where random atmospheric turbulence and thermal gradients distort a wavefront being received by the telescope. If the telescope is orbiting in space, varying dynamic and thermal stresses can also compromise its optical performance. An adaptive-optics system capable of correcting a distorted wavefront is therefore useful in enabling a large telescope to make full use of its resolving power.
Another application is in active phase conjugation, where the system is used to focus a laser beam through the atmosphere. An incoming wavefront is received and reflected from a selectively deformable mirror deformed to represent the phase conjugate of the incoming wavefront. This results in a reflected wavefront having a distortion impressed thereon that precisely compensates for the distortion in its propagation path, thereby providing a properly focused wavefront that maximizes power density when the wavefront reaches the target.
Prior art adaptive-optics systems typically use a deformable mirror for receiving thereon the incoming distorted wavefront; an interferometer for generating an interferogram indicative of the distortion present in the wavefront; a plurality of photodetectors for reading the interferogram at a corresponding plurality of locations; correction circuitry for computing a corresponding plurality of corrective signals; a corresponding plurality of integrating amplifiers; and a corresponding plurality of high voltage amplifiers for generating and applying varying high-voltage signals representing the corrections to a like plurality of actuator-like elements located in the mirror. The high voltages selectively deform the mirror to provide an outgoing wavefront that is free of the distortion present in the incoming wavefront. A prior art system of this type is disclosed in U.S. Pat. No. 3,923,400. Further prior art adaptive-optics systems, or elements thereof, are disclosed in the following U.S. Pat. Nos.:
______________________________________ U.S. Pat. No. 3,904,274 U.S. Pat. No. 3,766,415 U.S. Pat. No. 3,633,995 U.S. Pat. No. 3,758,199 U.S. Pat. No. 4,280,756 U.S. Pat. No. 3,660,777 U.S. Pat. No. 4,257,686 U.S. Pat. No. 3,567,325 U.S. Pat. No. 4,248,504 U.S. Pat. No. 4,379,697 U.S. Pat. No. 4,239,343 U.S. Pat. No. 4,383,763 U.S. Pat. No. 4,160,184 U.S. Pat. No. 4,322,837 ______________________________________
Prior art systems are restricted, however, by the realtime operation mandated by the random and rapidly changing nature of the atmospheric disturbances that distort optical wavefronts. Real-time operation will usually require that corrections be calculated and applied to the actuator elements within milliseconds of the wavefront being received on the mirror. This has usually dictated that the corrections be applied in parallel fashion to enable all the corrections to be calculated and applied within the short alloted time intervals. However, a prior art system applying corrections in parallel fashion requires a discrete amplification system for simultaneously calculating and generating high-voltage signals for each actuator element, a powerful digital computer for calculating the corrections in real time, and A/D and D/A hardware not required in a parallel all-analog system.
As is apparent with of prior art systems, the cost and complexity of a system increases as the number of actuators required increases. Because of these restrictions, prior art systems are currently limited to applications where about 200 or less actuator elements are required to accurately null the phase distortion in a wavefront. For atmospheric applications, this has restricted the use of prior art systems to telescopes having primary optics of about 30 cm or less in diameter. In phase conjugation, it has limited the ability of a deformable mirror to accurately focus a large-diameter laser beam incident thereon.